Treatment for cataracts and other diseases of the eye often requires removal of the natural lens form the eye. An artificial intraocular lens, in many cases, is surgically implanted to perform the visual focusing function of the removed defective natural lens. The intraocular lens (IOL) is composed of the optical lens body and a haptic device or element for positioning and securing the optical lens body in proper position within the eye. The lens body portion of IOLS in recent years has been formed of glass or polymeric materials that possess the appropriate optical qualities and that remain chemically and mechanically stable after implantation over a long period of time. The exact design of the haptic device depends upon the location within the eye at which the optic lens is to be placed. Three general designs are posterior chamber lenses, anterior chamber lenses, and iris plane lenses. The wide variety of IOL haptic element designs is shown in the literature, examples of which are Hoffer U.S. Pat. No. 4,244,060; Sheets U.S. Pat. No. 4,328,595; Feaster U.S. Pat. No. 4,418,431; Bayers U.S. Pat. No. 4,316,293; and Kelman U.S. Pat. No. 4,174,543 and U.S. Pat. No. 41,340,979.
A major difficulty with IOLs has been that the implantation process can cause serious injury to the corneal endothelium, which may result in corneal edema. Adhesive contact between the most commonly used intraocular lens body optical material, poly(methyl methacrylate) (PMMA), and the corneal endothelium during surgical implantation results in losses of endothelial cells that do not regenerate. Cell loss has been directly related to the number of times the intraocular lens contacts the endothelium during surgery, with approximately 20% loss resulting from each contact.
The relationships between cell adhesion and surface properties of the optic lens body such as surface energy, surface chemistry and surface rigidity have been studied by many investigators. Surface modification of the PMMA surface substrate has been shown to alter lens adhesiveness to cells. For example, Knight et al. in U.S. Pat. No. 4,170,043 describes reduced corneal endothelium damage for lenses coated with a water soluble film, such as polyvinyl alcohol, that is self-sacrificing in protecting the endothelium during implantation but dissolves away within 24 hours.
The literature discussing suitable IOL optic materials and implantation techniques reports that hydrophilic surfaces result in less cell adhesion damage during implantation than hydrophobic surfaces. Thus, much work is reported in which optic material surfaces are altered to achieve a more hydrophilic character than PMMA.
Investigators have used gamma radiation grafting to polymerize hydroxyethyl methacrylate (HEMA) and vinyl pyrrolidone (VP) onto a PMMA substrate. Using a laboratory "touch test" between the modified lens material and rabbit corneas, it was discovered that PMMA alone induces 10-30% damage, a PMMA/HEMA graft about 10% damage and a PMMA/VP graft less than 10% cell damage. However, these "touch tests" are relatively arbitrary and nonreproducible.
Another investigator studied silicone coated lenses, using as a test an 18-gram weight to press a sample intraocular lens and rabbit cornea together for 10 seconds to produce a consistent force on the endothelium. It was reported that PMMA caused "considerable damage", silicone resin lenses induced "less damage" than PMMA, and silicone elastomer created "far less damage". No quantitative comparisons of cell damage between the samples were possible.
In yet another study, the investigator constructed and employed an instrument directly measuring the force of adhesion between rabbit corneal endothelium and intraocular material samples. The average stress calculated for PMMA was 0.66 g/cm.sup.2 which was shown to be the highest of all materials studied. A plasma-deposited VP coating on PMMA and a conventional coating of Healon.TM. (manufactured by Pharmacia Inc.) on PMMA each lowered the stress to 0.19 g/cm.sup.2. Two hydrogels poly (HEMA) and Duragel.TM. (Soflex), exhibited the lowest stresses, 0.09 and 0.14 g/cm.sup.2, respectively, of the materials tested.
Thus, the best state-of-the-art coatings discovered prior to the present invention appear to result from surfaces that are soft, hydrophilic hydrogels. However, hydrogel surfaces, such as HEMA and VP, while demonstrating lower cell damage relative to the PMMA substrate and other coatings tried, exhibit a number of disadvantages. For example, the coatings are soft and easily damaged. Also, they are difficult to package and difficult to hydrate properly at surgery. Further, hydrogels are prone to calcification and bacterial contamination. Thus, the IOL constructions known prior to the present invention may still cause significant damage during implantation or may be otherwise unsuitable for general use.